This invention relates to a method of generating a digital road map, in particular, a technology of mapping a lane central line onto a road map by utilizing a driving trajectory of a vehicle.
The integration of a digital road map and a precise positioning system is extremely significant for an advanced driving safety application which draws increasing attention as, for example, a next-generation vehicle navigation system. However, current digital road maps hardly satisfy the requirements of a driving safety application in terms of accuracy and information content. Therefore, there are increasing needs of mapping roads onto a map at high accuracy and in a detailed manner.
Conventional road mapping technologies are based on data measured by survey methods which can be roughly classified into two categories: a ground-based survey and an air survey. A ground-based survey system includes in-vehicle equipment (including a GPS receiver). Devices such as a camera (which may be replaced by a laser scanner), a display device, and a communication device may be connected to the in-vehicle equipment. Such a system acquires detailed information on a road on which a system-carrying vehicle travels. However, the application range of the system is limited because of problems such as the limited applicability of the system only to an extremely small range of survey, a long cycle required to collect data, complicated data processing after the survey, and a high cost for system maintenance.
On the other hand, the air survey appears to be a promising solution for its large coverage area and quick data collection. However, efficient extraction of necessary information from complicated data suffers great technical difficulties. Moreover, the generation of a highly accurate road map based on the conventional survey data requires resources and time.
FIG. 23 illustrates a process performed by a vehicle navigation system which uses an in-vehicle GPS device and a digital road map database. The vehicle navigation system determines the location of a vehicle on a map. This process is referred to as map matching. Location information of the vehicle is determined based on a signal captured by the in-vehicle GPS device. As illustrated in FIG. 24, when a GPS positioning signal is available, the location information acquired by the GPS device is checked against a digital map to enable the use of the vehicle navigation. However, since the GPS signal is a radio wave coming from a GPS satellite, the GPS signal is unavailable in some cases. In the urban areas, in particular, positioning using the GPS cannot be used at a location where radio waves coming from a predetermined number of satellites cannot be received due to sky occlusion. Therefore, various complementary methods such as positioning using a ground wave, a pseudo GPS satellite system, and a radio identification tag have been developed as methods of determining a precise location of the vehicle.
For the current digital road map, the relative precision of a link of nodes provided on a road is regarded as more important than the accuracy of road geometric information for its navigation use. For example, as illustrated in FIG. 25, a link AB allows a driver to know that a vehicle gets to a point B after passing through a point A. This route guidance function is called navigation. For a navigation application, whether or not the link AB precisely indicates a road between the point A and the point B is not always important. For the vehicle navigation, a simplified road map functions effectively. For a driving safety application in which a vehicle is required to be guided precisely onto a lane, however, whether the link AB on the road map is a straight line AB or a curve ACB makes a significant difference.
An advanced driving safety assistance system is one of the most promising technologies in the development of the next-generation vehicle navigation system. For example, the early researches such as the U.S. Department of Transportation project, Enhanced Digital Mapping (EDmap) as described in Non-Patent Document 1 and Non-Patent Document 2 and the European Union counterpart, NextMap, as described in Non-Patent Document 3 indicate that the integration of an enhanced digital road map and a precise positioning system is extremely important for the driving safety application. In the systems described above, applications using a map or enhanced applications such as Curve Speed Assistant-Warning and Control (CSA-W and CSA-C) have been subjected to a verification test. The applications match a vehicle position with a map to predict the curvature of a road ahead to electronically visualize a road that the vehicle is approaching. The DaimlerChrysler AG (currently, Daimler AG) proposes another application for Lane Following assistant-Warning (LFA-W). The application combines a vision lane tracker system, a lane-level digital map, and a highly accurate positioning system to warn a driver when a vehicle is about to leave the lane.
Corresponding to the positioning accuracy of the in-vehicle GPS device in the present days, the current digital navigation road map provides location information of a road only at the level of granularity with an emphasis on topological correctness of road-node links. Since the vehicle navigation system uses the matching of the location information of the vehicle to the map, the vehicle navigation system is less sensitive to the accuracy and the contents of the road map. On the other hand, the driving safety application requires extremely precise road geometric information, for example, a road central line, a lane central line, a road curvature and the like, and in addition, the detailed contents, for example, the number of lanes, a stopping location, and a speed limit. The generation of the enhanced digital road map required for the driving safety application by a conventional method needs a high cost. The ground-based survey is laborious, whereas the air survey suffers a problem of technical difficulties in information extraction. The probe car survey provides a method of acquiring detailed information of the road. However, the application of the survey with the probe car is limited to a large-scale survey because a high cost is required. Consequently, in order to satisfy a variety of demands for the enhanced digital road map, it is important to provide a cost-effective method.
Further, the current digital road maps scarcely satisfy the requirements of the driving safety application in view of the accuracy and the contents of information. Moreover, conventional road map generation technologies are resource-and time-consuming. In regard to the problems described above, this invention provides an appropriately cost-effective method of generating a highly accurate road map.
For example, as illustrated in FIG. 23, it should be noted that the result of matching of GPS location information to a map for the vehicle navigation has been simply abandoned. The result of matching of an acquisition sequence of the GPS location information acquired by the in-vehicle GPS device and the corresponding map (the result is referred to as GPS track) contains a trajectory of a vehicle motion, which provides perfect matching with road geometric information. The GPS track data is not sufficient to represent precise road information because of an error deviation of a GPS positioning signal. However, the collection and the accumulation of a large number of pieces of the GPS track data as described above allow road information to be statistically derived from the GPS tracks. This invention provides a method of collecting and accumulating such GPS track data which has been conventionally abandoned.
An object of this invention is to provide a novel method of statistically analyzing data obtained from an in-vehicle GPS device to dynamically generate a highly accurate road map. The basic idea of this invention comes from a technical concept called “data recycling”. According to the concept of “data recycling”, a large number of pieces of track data obtained by the in-vehicle GPS device are collected because the track data obtained by the in-vehicle GPS device contains road-specific information. Further, this invention provides a map generation method at a lower cost than that required by a conventional method.